This invention relates to a two-dimensional solid image-pickup device the specific feature of which resides in the reading of signals.
In general, a solid image-pickup element is provided by forming photo-detectors and a scanning mechanism on a semiconductor material such as a silicon substrate, and it can sense images in the frequency range of from visible rays to infrared rays. The solid image-pickup element is small in size, small in weight and high in reliability, when compared with the conventional image-pickup tube. Furthermore, in manufacturing the element, the number of points to be adjusted is very small. Therefore, many industrial users are interested in the solid image-pickup element.
The scanning mechanism of the solid image-pickup device mainly employs MOS switches or charge coupled devices (hereinafter referred to merely as "CCD's", when applicable). In the noise former scanning mechanism, the spikes, which are caused when the MOS switches are operated to read signals, are mixed with the signals, thus lowering the S/N ratio. The noise spikes differ according to the reading lines, which forms a so-called "fixed pattern noise", thus further lowering the S/N ratio. Accordingly, the former scanning mechanism is not applicable to the case where weak signals should be detected with a high S/N ratio. On the other hand, in the latter scanning mechanism, especially in the interline CCD system which is widely used because photo-detectors can be freely selected similarly as in the aforementioned MOS system, the CCD's are arranged between the lines of detectors. Accordingly, in order to increase the effective areas of the detectors, it is desired that the area of the CCD section is as small as possible. On the other hand, the CCD's charge transferring capability is proportional to an accumulated gate area per stage of CCD's. Accordingly, decreasing the area of the CCD section means that the maximum amount of charge handled is limited. These are serious problems in detecting a small signal in a large background, as in the case of an infrared solid image-pickup element.